The present invention is directed to a method and apparatus capable of monitoring raster positioning errors (e.g., those produced by photoreceptor motion error, polygon wobble or optics vibration in a raster output scanning printing system) and altering the output signal in order to compensate for such error. Raster position errors as small as 0.5% give rise to visually objectionable banding in halftone output prints. Such errors typically arise in raster scan image output terminals (IOTs) due to polygon wobble and photoreceptor velocity non-uniformity. Although techniques have been proposed to eliminate such error, it is typically expensive to control or limit the error to an acceptable level; a level below which the error will not be detected by the unaided eye.
Heretofore, a number of patents and publications have disclosed techniques for compensating for position errors in image output terminals, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 4,073,566 to Noguchi et al., issued Feb. 14, 1978, discloses the use of a stationary hologram in the optical path of a beam scanning device in order to compensate for displacement of the beam resulting from errors in parallelism of the facets of a polygon mirror.
U.S. Pat. No. 4,707,122 to Lama et al., issued Nov. 17, 1987, discloses the use of an optical filter in the optical path of a reprographic machine to eliminate the effects of mechanical vibration that result in strobing (exposure modulation). The filter is tuned so as to have an incidence profile that is a function of the vibration frequencies.
U.S. Pat. No. 4,801,978 to Lama et al., issued Jan. 31, 1989, teaches an electronic printer employing an image write bar for exposing a photoconductive member. An encoder is used to monitor the vibration of the rotating photoconductive member, the signals generated by the encoder being employed to modify the on/off timing and/or the intensity of the image bar output.
U.S. Pat. No. 4,884,083 to Loce et al., issued Nov. 28, 1989, discloses a printing system employing a raster output scanning device that is compensated for the effects of motion of the medium that it is used to expose. The system disclosed is directed toward compensating for periodic exposure modulation in images transmitted by the raster output scanning device to the surface of a photoreceptor.
U.S. Pat. No. 4,332,461 to Cail et al., issued Jun. 1, 1982, teaches a servomotor system capable of monitoring and compensating for motion error in a continuously variable reduction copier. The copier employs a light-lens scanning carriage driven in relationship to the rotation of an image carrier.
As described by R. Loce, W. Lama, and M. Maltz in "Modeling Vibration-Induced Halftone Banding in a Xerographic Laser Printer," Journal of Electronic Imaging, Vol. 4 No. 1, p. 48-61, the relevant portions being hereby incorporated by reference, in a raster scanning printer, a laser beam is scanned across a photoreceptor in a direction perpendicular to the photoreceptor motion. When there is vibratory motion of the photoreceptor or wobble in the polygon mirror, the raster lines on the photoreceptor will not be evenly spaced. The authors analyze the positioning error and show that fractional raster spacing error is equal to photoreceptor fractional velocity error. The raster position errors result in various print defects, of which halftone banding is the dominant defect.
Polygon wobble in laser scanners is described in "Laser Beam Scanning-Opto-Mechanical Devices, Systems and Data Storage Optics," Marcel Dekker, Inc. (1985), p. 78. A possible correction technique for such wobble is described by L. Mailloux in "Exposure Compensation for Polygon Wobble Errors," Xerox Disclosure Journal, Vol. 14, No. 3, May/June 1989, where it is suggested that exposure control may be employed to reduce the resulting error.
Also, several authors have considered the effects of nonuniform photoreceptor motion and other sources of noise in digital printers, including Bestenreiner, Geis, Helmberger, and Stadler, "Visibility and Correction of Periodic Interference Structures in Line-by-Line Recorded Images," J. Appl. Phot. Engr. Vol. 2, p. 86-92 (1976); Takiguchi, Miyagi, Okamura, Ishoshi, and Shibata, "Effect of Photoreceptor Drum Rotational Speed Variation on Laser Beam Printer Halftone Reproduction," Proceedings of the SPSE Third International Congress: Recent Advances in Non-Impact Printing Technologies, p. 168-172, San Francisco (Aug. 1986). Haas "Contrast Modulation in Halftone Images Produced by Variation in Scan Line Spacing," J. Imaging Tech. Vol. 15, p. 46 (1989), examined the effects of periodic scan line position errors when printing periodic binary patterns (e.g., halftones). Bloomberg and Engeldrum, "Color Error due to Pixel Placement Errors in a Dot Matrix Printer," Proceedings of the SPSE Third International Congress: Recent Advances in Non-impact Printing Technologies, p. 257-260 (Aug. 1986), analyze color error on a print that is caused by random pixel placement errors. Loce and Lama in "Halftone Banding due to Vibrations in a Xerographic Image Bar Printer," Journal of Imaging Technology, Vol. 16, No. 1, p. 6-11 (1990) and in "Halftone Banding Due to Vibrations in a Xerographic Image Bar Printer," SPSE 41st Annual Conference, Washington (1988), employ exposure and xerographic models to examine vibration induced halftone banding in image bar printers.
In "Color Errors Due to Pixel Placement Errors in a Dot Matrix Printer," Third International Congress on Advances in Non-impact Printing Technologies--Society of Photographic Scientists and Engineers, (Aug. 24, 1986), pp. 257-260, S. Bloomberg teaches the modeling of a dot matrix printer using a structured dot theory. The model was used to study the impact of pixel placement errors on color.
In accordance with the present invention, there is provided a raster output scanning printer, including: an exposure device suitable for emitting a light beam therefrom; a movable imaging member; a rotating, multifaceted polygon suitable for reflecting the light beam toward a photoresponsive surface of said imaging member; a controller for controlling the velocity of the photoresponsive surface about a nominal velocity; means for monitoring the velocity of the photoresponsive surface and determining the deviation thereof from the nominal velocity; means for determining the deviation of the position of the light beam from a nominal position on the photoresponsive surface; first compensation means, responsive to the velocity deviation of the photoresponsive surface, for producing a first motion compensation signal; second compensation means, responsive to the light beam position deviation, for producing a second motion compensation signal; a summing circuit for summing the first motion compensation signal and the second motion compensation signal to produce a cumulative error signal; and means for altering the exposure level produced by the light beam in response to the cumulative error signal to expose the imaging member in a manner so as to compensate for the motion error arising from the velocity and light beam deviations.
In accordance with another aspect of the present invention, there is provided a raster printer, including: a movable imaging member; an exposure device suitable for raster-wise exposing a surface of the imaging member and thereby creating a latent image thereon; a plurality of position deviation detection means, each generating a compensation signal representative of the deviation of an exposed raster from a nominal position on the surface of the imaging member; summation means, responsive to the compensation signals generated by said plurality of position deviation detection means, for producing a cumulative error signal; and means for altering the exposure level of said exposure device in response to the cumulative error signal to expose the surface of the imaging member in a manner so as to compensate for the cumulative error.
In accordance with yet another aspect of the present invention, there is provided a method for automatically compensating for raster position deviations in a raster printing system, including the steps of: predicting an intensity variation for a raster exposed on an imaging member where the intensity variation is caused by each of a plurality of sources of motion error within the raster printing system summing the intensity variations from each of the plurality of sources of motion error; and altering, in response to the summed intensity variation, the level at which each pixel of a subsequent raster is exposed on the imaging member to compensate for the motion error.
One aspect of the invention deals with a basic problem in the variance or deviation of raster placement on a photoconductive surface in order to print an output image. This aspect is further based on the discovery of an inexpensive technique that alleviates cumulative positional error arising from multiple sources (e.g., photoreceptor motion nonuniformity and polygon wobble). The technique alters the local exposure level of pixels within a raster based upon the error in positioning of the raster with respect to a nominal raster position. The local exposure level may be altered by either varying the intensity of a exposure device or by altering the exposure period with a constant intensity device (effectively varying intensity).
The technique described herein is advantageous because it is inexpensive compared to other approaches. In addition, it can be used to correct for positional errors no matter what the source, so long as the error can be characterized or monitored and the plural sources can be summed.